The U.S. Marine Corps says it has recovered the remains of nine of the 12 Marines killed when two helicopters crashed during nighttime training off Hawaii.

Remains of three Marines were not recovered, the Marine Corps said Thursday in a statement. Sgts. Dillon Semolina and Adam Schoeller and Cpl. Christopher Orlando are still missing.

The Marines say they have completed recovery operations.

The helicopters were part of Marine Heavy Helicopter Squadron 463 based at Kaneohe Bay. The Coast Guard initially reported that the choppers had collided, but the Marines said later it wasn't known whether there was a collision. The cause remains under investigation.

It happened off Oahu's North Shore in January. The Marine Corps says teams mapped the debris site and recovered significant wreckage from both helicopters.